1. Field of the Invention
This invention relates to means and methods for filtering selected wavelengths by resonant scattering. More particularly the invention relates to filtering selective wavelengths by mismatching the index of refraction of a host material and a scattering-absorbing material.
2. Prior Art
Numerous optical filters, including filters for protecting against laser radiation have been disclosed. For example Hutchinson, et al in U.S. Pat. No. 3,519,339 discloses protective laser goggles comprised of front and rear glass plates for respectively absorbing ultraviolet light and infrared light, and including sandwiched therebetween two separate multilayer dielectric coatings made up of high and low index of refraction materials to define a narrow pass band of visible light which is different from the wavelength of a particular laser light.
Sarna in U.S. Pat. No. 3,792,916 discloses a filter assembly for selectively removing individual emission lines of laser energy from the visible spectrum comprising at least one pair of Fabry-Perot filters which transmit laser energy emissions for dissipation in the filter assembly and reflect harmless radiation for transmission through the assembly.
Richards, et al in U.S. Pat. No. 4,070,101 discloses a wide angle narrow bandpass optical filter including multiple absorption materials in a common path. Langberg in U.S. Pat. No. 3,190,172 employs a transparent scattering cell containing sodium vapor. The absorption of spectral radiation produces a resonance radiation (i.e., the absorption of photons of a specific wavelength and the re-emission of photons of the same or other wavelengths) within the cell.
The standard Christiansen filter is comprised of two materials which have different dispersion curves that intersect at one point. At the intersection, the indices of refraction of the materials is the same and the filter transmits light at the frequency corresponding to the point of intersection with little loss, while significantly scattering light at other frequencies.
Problems remain with all the prior art filters. First, it is difficult to obtain a sufficiently narrow absorption band in an absorption filter, so that out-of-band transmissions are high enough to not compromise the effectiveness of optical equipment or an operator of a device. Second, prior art filters are relatively large, of fixed configuration and relatively rigid. This implies difficulty in integrating the filter into most existing optical elements. Third, it is desirable to increase the hardness to laser radiation of prior filters without materially increasing size or complexity.
Theoretical calculations using Mie scattering theory have been made for three dimensional scattering in paint coatings due to small spherical scattering particle dispersed in the coating. The results are published in "Application of Mie Scatter Theory to the Reflectance of Paint-Type Coatings", R. N. Schmidt, P. M. Treaenfals and E. J. Meeham Proceedings of the ASME Fourth Symposium on Thermophysical Properties, 1968 at p. 256 and are incorporated herein by reference.
FIG. 1 is qualitative graph illustrating some general conclusions of the Schmidt, et al report. In FIG. 1, Ks is a scattering coefficient which is a partial measure of the influence of a particle in a paint coating on light incident on the coating. Ks is a measure of the scatter of light striking such a particle. Ks is defined as: assuming energy of uniform intensity is incident on a particle and surrounding area, it is the ratio of energy scattered (i.e, deviated by reflection or refraction) to that going through an area equal to the geometrical cross-section of the particle. The ratio of particle radius to wavelength is represented by .alpha., where: ##EQU1## where .lambda. is the wavelength in the surrounding media and Rg is the geometrical radius of the particle.
FIG. 1 is plot of Ks versus .alpha. (shown as line 10) with the index of refraction no of the host material equal to a constant Q. Two peaks 12 and 14 are shown as well as points 16 and 18. Assuming .lambda. is fixed, between the origin O and the value of .alpha. corresponding to the first peak 12 (i.e., .alpha. 1) a smaller .alpha. implies a smaller Rg particle. This is shown by noting that .alpha. 2 corresponding to point 16 is larger than .alpha. 3 corresponding to point 18.
As Rg becomes larger (i.e., generally as .alpha. exceeds .alpha. 1) the Schmidt, et al paper discloses that scattering is shifted from predominately side scatter to predominately forward scatter. The Schmidt, et al paper was concerned with properties of reflecting paint coatings. The present invention discloses a new optical filter based upon Mie scattering theory.